Integrating Different Age Groups

I have 2 groups of peachicks that I want to integrate and overwinter and I am not sure how to do it. The first group (6 of them) is older, born on the 23rd of June, so about 3 months old. The second group consists of 3 singleton chicks ranging in age from 1 up to 2 months old. Right now they are sharing the shed (10X20) but the singleton group are in an x-pen inside so the bigger ones can see them but not get at them.
Would they be alright if I just let them mix? There is considerable size difference between the oldest and the youngest. Do I wait until they are a little older?
Thanks